A display device, such as a liquid crystal display device and an organic EL (electroluminescence) display device that are thinner and lighter than a CRT, has been used as the monitor of a portable terminal such as a cellular phone and a mobile device or as the monitor of a notebook personal computer. An image is displayed on a liquid crystal display device or an organic EL display device by forming a display part, which has pixels arranged in a matrix, on an insulating substrate such as a glass substrate using the thin film formation technology and by sending signals, corresponding to video data, to the pixels via the driver circuits such as an externally installed gate driver and a data driver to control the orientation of liquid crystals or the luminescence of organic EL devices. A recent development in the thin film formation technology makes it possible to form polycrystalline silicon TFTs on the same substrate as that of the display part and therefore to form a part of the driver circuit with polycrystalline silicon TFT circuits.
Compactness, low power dissipation, and high performance are important for a portable terminal and, to satisfy those needs, there is a need for a compact, low-power-dissipation type display device. To make a display device compact, the display part and the driver circuit are integrated into a glass substrate to reduce the number of external parts and to make the device compact. The integration of the display part and the driver circuit reduces the load capacity that would be caused by a connection resistance or a wiring(interconnection) to external connection terminals required in case of the driver circuits mounted external to the display part and, at the same time, reduces power dissipation. Recently, a display device is required to display a high definition, clear image, with increasing demand for an active matrix display device that forms each pixel independently. An active matrix display device has switching devices, at least one for each pixel. When a switching element is turned on by the signal corresponding to an image supplied from the driver circuit and the signal controlling a switching element, the signal corresponding to an image is sent to each pixel for display. When an active matrix display part and the driver circuit are integrated, for example, on a glass substrate, the switching elements (TFTs) of the pixels and the TFTs of the driver circuits formed on the same substrate are fabricated at the same time.
The TFTs described above include two types of TFT, n-channel type and p-channel type. Because a polycrystalline silicon film, which is an active layer, usually tends to be of n-type, an n-channel TFT is slightly of depression type, as a result of which, driving power is relatively increased and off current is increased. For a display device, especially, a display device used for a mobile communication terminal, a low off current is required at least in a switch TFT to reduce power dissipation. Therefore, doping is performed in the channel region of an n-channel TFT during TFT fabrication process to control a threshold voltage VT.
Because channel doping is usually performed for the channel regions of a plurality of TFTs at a time, the dose of dopant implanted into the plurality of TFTs is almost equal among them. It is also possible to change the dose according to TFTs during a single doping operation. A reference is made to a publication such as Japanese Patent Kokai Publication JP-A-8-264798.
Japanese Patent Kokai Publication JP-A-8-264798 (pages 4-7, FIG. 5) discloses a method of varying the thickness of a control film (silicon dioxide film), provided for controlling the dopant implantation amount, and then performing doping from above the control film according to the region with a large dosage in the thin control film part and a small dosage in the thick control film part.